QMAINWINDOW

• 为什么在头文件里不用预声明QSTRING,QMENU跟QTOOLBAR？
• QMainWindow has its own layout to which you can add QToolBars, QDockWidgets, a QMenuBar, and a QStatusBar. The layout has a center area that can be occupied by any kind of widget.
• The closeEvent() function is a virtual function in QWidget that is automatically called when the user closes the window.

          在程序中用PROTECTED的原因是：它在QWidget中是PROTECTED类型的